Antenna



Sept. 22, 1953 LAN JEN CHU ANTENNA 2 Sheets-Sheet 1 Filed July 9, 1945 INVENTOR LAN JEN CHU ATTORNEY Sept.v22, 1953 LAN JEN cHu ANTENNA "2 Sheets-Sheet- 2 Filed July 9, 1945 FIG. 3

. PARABOLOID REFLECTOR 50 INVENTOR LAN JEN CHU ATTORNEY Patented Sept. 22, 1953 ANTENNA Lan Jen Chu, Brookline, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application July 9, 1945, Serial No. 604,021

7 Claims.

This invention relates to antennas for high frequency communication systems and particularly to means for obtaining an unsymmetrical radiation pattern. More specifically, the invention is directed to a reflector shaped or modified to pro.- duce a radiation pattern in which the energy distribution closely approximates the relationship csc a in one plane; generally a vertical plane, where 0 is the angle measured from the axis of the undistorted or symmetrical beam (i. e., with an undistorted or conventionally shaped type of reflector) towards the modified side of the reflector.

In certain radio object locating systems, for example, ground search apparatus or low-altitude airborne sets, it is desirable that the distribution of energy in a vertical plane be such that the variation of energy density versus the radiation angle approximates a cosecant-squared function.

This distribution provides generally uniform echo strength for targets disposed at substantially equal elevations with respect to the antenna, regardless of whether the target is close in or distant, and it eliminates, to a great degree, the need for tilting the antenna to obtain satisfactory coverage of the area under observation.

The present invention contemplates an economical method of modifying a usual type of parabolic reflector, such as one shaped as a parabolic cylinder or as a paraboloid, to obtain a different type of energy distribution pattern, such as a pattern of csc 0 configuration.

Accordingly, one of the objects of the invention is to afford an unsymmetrical distribution pattern of radiant energy, for example a cosecantsquared pattern, by modification of a usual type of parabolic reflector.

Another object of the invention is to provide a parabolic reflector having a portion of its reflecting surface suitably shaped to produce a radiation pattern of any desired configuration.

Other objects, features, and advantages of this invention will become apparent from the following descriptiontaken in connection with accompanying drawings in which:

Fig. 1 is a perspective view of an antenna showing one embodiment of the invention;

Fig. 2 is a diagrammatic side view showing the radiation patterns produced by an unmodified reflector and a modified reflector according to the invention respectively;

Figs. 3 and 4 are enlarged side sectional views I of a modified parabolic reflector according to two embodiments of the invention illustrating the reflection of energy rays therefrom;

2 Figs, 5 and 6 are diagrams for use as a basis to illustrate the mathematical analysis of the invention;

Fig. '7 is a graph illustrating the energy power relation of the antenna according to the invention; and V Fig. 8 is a side sectional View of a modified paraboloidal reflector according to another embodiment of the invention.

In general, the antenna according to this invention, comprises a radiating source, which may be a linear array, a pillbox type or generally a point source type of radiating source, for illuminating a reflector having a portion of its reflecting surface suitably shaped to produce an unsymmetrical energy distribution pattern. The reflector may be shaped substantially as a paraboloid of revolution, a semi-parabolcid, or as a hyperbolic, or parabolic cylinder, or as a half parabolic cylinder. The type of radiating source used will depend on the general shape of the reflector. For a paraboloid, a point source such as a dipole or horn type of radiating element may beused. For a hyperbolic or parabolic cylinder the radiating source may be for the linear array type such as those described in the copending application of Luis W. Alvarez, Serial No. 542,287, filed June 27, 1944, and Serial No. 509,790, filed November 10, 194.3, and of Jack Steinberger, Serial No. 585,227, filed March 22, 1945, or of the pilibox type. A pillbox type radiator comprises a parabolic cylindrical reflecting surface of small axial dimension mounted between a pair of plane parallel metallic walls or plates which serve to enclose the ends of the cylinder.

By way of example and for simplicity of description, the invention is hereinafter more fully described with reference to the preferred embodiment in which the reflector is shaped as a parabolic cylinder. Accordingly in Fig. 1, there is shown a reflector Hi having the major portion of its reflecting surface shaped as a parabolic cylinder illuminated in a conventional manner by a radiating source ll of the linear array type such as disclosed in the above mentioned copending applications. As thus far described the antenna is adapted to direct an energy beam along a main path to produce a pencil type beam having a symmetrical pattern as indicated in dotted lines in Fig. 2. The antenna comprising reflector It and radiator ll may be supported and mounted in any suitable and well'known manner.

As shown in Fig. 1, a portion l3 near and along the upper edge portion of reflector it is bent,

: rolled, or flared backwardly away from source ll according to one embodiment, but may if desired be bent forwardly to give a modified non-parabolic shape to the reflecting surface. This changed configuration is adapted to direct a portion of the energy impinging thereon along a divergent path whereby an unsymmetrical radiation pattern is produced. The portion (3 may be shaped according to geometric optics so that the radiation pattern produced by reflector l9 varies substantially as the square of the cosecant of the angle measured from the axis of the symmetrical pattern, as indicated by the solid line configuration in Fig. 2. The non-parabolic portion l3 may be located along the upper or lower portion of reflector 10 depending on whether it is desired to direct the resultant beam pattern upwardly or downwardly respectively. Fig. 3 indicates the manner in which the rays of energy from source F are reflected by the reflector I with the modified portion l3 bent backwardly, line [4 designating the normal parabolic surface.

Fig. 4 shows a modification of the invention wherein the reflector has a semi-paraboloidal or semi-parabolic cylindrical reflecting surface l0, such as one formed by cutting the reflector approximately along the line of intersection 12 with the surface of a plane passed through the vertex and focus or focal line. In Fig. 4 this line of intersection, which may be called the vertex line, is shown as the lower edge of the semi-paraboloidal or semi-parabolic cylindrical reflector. Preferably the portion 13' along and adjacent the lower edge of surface In is bent or curved inwardly toward source F, although it will be apparent that the upper or peripheral edge portion of surface Hi may be bent inwardly according to the invention. The arrows indicate how the energy rays from source F are reflected by surface l G. For simplification, the reference to portion H3 in the present description willbe understood to include portion I3.

In a similar manner, as shown in Fig. 8, a

paraboloidal reflector 10 may be shaped to afford an unsymmetrical pattern including one of csc e configuration, by bending back a desired section l3 of the peripheral edge portion of the paraboloid. In this figure, dashed line I4 desig.

nates the normal, unmodified paraboloid surface, F designates the source of energy applied to this reflector, and the line l2 between F and the surface of said reflector represents, as in Fig. 4, the vertex line of said reflector.

The shaping of portion IE to obtain the desired pattern may be determined by experimentation or by means of the following mathematical analysis which will aid the understanding of the invention, it being assumed that a line radiating source and a cylindrical reflector such as described hereinbefore is used as a basis and the effect of diffraction is neglected. It is to be understood however that a similar analysis may be applied to a paraboloidal reflecting surface. In Fig. a ray of radiant energy FA extends from the feed F to the point A, making an angle #1 with the horizontal axis. At A it is reflected in the direction AP at an angle 0 with the axis AB. The normal AN makes an angle 5 with the axis. According to the geometric law of reflection the angles of incidence and reflection are each equal, so that fi=l /z(+0) (1) and B= Yaw-0) 2) Let I(/) denote the angular intensity distribution of the radiation from the feed and let P(0) denote the intensity or power as a function of 0 after reflection. The total power in the two patterns must be equal. Therefore:

Where (1112"1/4) is the angle subtended by the reflector at the feed and 02 and 01 are the maximum and minimum angles of the polar diagram.

When the lower limits of the last mentioned Equation 3 are made'variable they fix the relationship between v.0 and 0 providing there is a one to one correspondence. For example, in Fig. '7 there is plotted as a straight line the graph P: f I ea for the case in which the primary pattern 10b) is a constant, K. The curved line is the value of g mma for the pattern P(0)=csc 0. Since the two integrals are equal according to Equation 3, the value of 0 corresponding to any given value of t may be found by following the dashed line. In this particular example, the Equation 3 reduces to:

Since Imp) usually depends on experiment, its integral may in general be approximated by using a planimeter or some other method of approximate integration, such as Simpsons rule. Once 0 is obtained in terms of o, the use of Equation 2 fixes the orientation of the surface portion 83.

In Fig. 6,

an 2 I! where (ll +0) /2 is the angle of incidence, Equation 1, and r is as shown in Fig. 6. On integrating, Equation 5, v

log -=f n i (6) This gives the distance and shape to the surface portion E3 in terms of 1/1 since 6 is already a function of #1. Two solutions are possible depending on whether the reflected rays diverge as in Fig. 3 or converge as in Fig. 4. The corresponding value of dfl/dgl/ will be positive or negative accordingly.

The gain of a modified reflector according to the invention is due to that portion of the reflected wavefront which is in phase to within A/l. In the case of the 030 0 pattern this region contains two-thirds of the total energy, the remaining third of the energy being used for the flared part of the beam. Thus there are two factors which reduce the gain below that of a pencil type beam: (0a) the two-thirds factor inherent in the 08029 pattern, (b) the fraction of may be made therein without departing from the invention.

What is claimed is:

1. An antenna comprising a radiating means and a reflector, said reflector having more than half of its reflecting surface shaped as a parabolic cylinder adapted to direct radiant energy along a main path in a substantially symmetrical radiation pattern, a relatively small portion of the reflecting surface near and along the length of one of the edges parallel to the longitudinal axis of said reflector being of non-parabolic shape and adapted to direct said energy along a divergent path, whereby said reflector produces an unsymmetrical radiation pattern which varies substantially as the square of the cosecant of the angle measured from the axis of the symmetrical pattern.

2. An antenna as claimed in claim 1 wherein said non-parabolic portion of said reflecting surface is bent backwardly away from said radiating means.

3. An antenna as defined in claim 1 wherein said radiating means is a linear antenna array disposed along a line parallel to said longitudinal axis.

4. An antenna comprising a reflector and radiating means for directing radiant energy onto said reflector, said reflector having more than half of its reflecting surface shaped as a paraboloid of revolution adapted to direct energy along a main path in a substantially symmetrical radiation pattern, said reflector having a relatively small portion of its reflecting surface along and near an edge portion bent backwardly away from said radiating means to provide a non-parabolic portion of such shape that it is adapted to direct energy along a divergent path, whereby an unsymmetrical radiation pattern, which varies substantially as the square of the cosecant of the angle measured from the axis of the symmetrical pattern, is produced by said reflector.

5. An antenna comprising radiating means and a reflector, said reflector having more than half of its reflecting surface shaped substantially as a semi-parabolic cylinder adapted to direct radiant energy along a main path in a substantially symmetrical radiation pattern, a relatively small portion of the reflecting surface near and adjacent the edge along the vertex line thereof being bent toward said radiating means to provide a non-parabolic portion of such shape that it is adapted to direct energy along a divergent path whereby an unsymmetrical radiation pattern which varies substantially as the square of the cosecant of the angle measured from the axis of the symmetrical pattern is produced by said reflector.

6. An antenna comprising radiating means and a reflector, said reflector having the greater portion of its reflecting surface shaped substantially as half a paraboloid of revolution adapted to direct radiant energy along a main path in a substantially symmetrical radiation pattern, a relatively small portion of the reflecting surface near and adjacent the edge along the vertex line thereof being bent toward said radiating means to provide a non-parabolic portion of such shape that it is adapted to direct energy along a divergent path whereby an unsymmetrical radiation pattern which varies substantially as the square of the cosecant of the angle measured from the axis of the symmetrical pattern is produced by said reflector.

7. An antenna comprising a parabolic reflector and radiating means for directing radiant energy onto said reflector, said reflector having more than half of its reflecting surface adapted to direct radiant energy along a main path in a substantially symmetrical radiation pattern and a relatively narrow edge portion of non-parabolic shape adapted to direct said energy along a divergent path whereby an unsymmetrical radiation pattern which varies substantially as the square of the cosecant of the angle measured from the axis of the symmetrical pattern is produced by said reflector.

LAN JEN CHU.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,798,083 I-Ialback Mar. 24, 1931 2,032,622 Guillou Mar. 3, 1936 2,261,733 Pahl Nov. 4, 1941 2,270,965 Peterson Jan. 27, 1942 FOREIGN PATENTS Number Country Date 349,043 Great Britain Nov. 13, 1929 770,482 France July 2, 1934 

